A fluid catalytic cracking (FCC) is a process for the conversion of straight-run atmospheric gas oil, vacuum gas oil, certain atmospheric residues, and heavy stocks recovered from other operations into high octane gasoline, light fuel oils, and olefin-rich light gases. In simplified terms, the cracking reactions are carried out in a vertical reactor vessel in which vaporized oil rises and carries along with it, in intimate contact, small fluidized catalyst particles. The reactions are very rapid, and only a few seconds of contact time are necessary for most applications. In a petroleum refinery, the FCC unit typically processes 30-50% of the crude oil charged to the refinery and produces gasoline blending components which may account for 50 to 20% of the total motor gasoline produced in the refinery.
Every FCC unit comprises a reactor and regenerator where the feedstock is cracked into a reactor effluent containing hydrocarbons. The hydrocarbons range in composition from the lightest methane to the heaviest, highest boiling hydrocarbon component in the feedstock. The reactor effluent also includes hydrogen and impurities such as hydrogen sulfide, hydrogen cyanide, and nitrogen compounds. The reactor effluent is passed to a main fractionator which separates the reactor effluent into wide boiling ranges such as an overhead stream which includes gasoline and lighter components and liquid products such as heavy naphtha, cycle oils, and slurry. The overhead stream is passed to a gas concentration unit or to an unsaturates gas plant wherein the unstable gasoline components and lighter components are separated into a fuel gas stream, a C.sub.3 -C.sub.5 olefinic hydrocarbon stream, and a stabilized, motor fuel, or gasoline blending component. Chapter 3.3 of the "Handbook of Petroleum Refining Processes, 2.sup.nd Edition, edited by Robert A. Meyers, and published by McGraw Hill, N.Y., in 1997, which is hereby incorporated by reference, describes the FCC unit and the gas concentration section. The operation of a gas concentration section for the recovery of gaseous hydrocarbons from gasoline disclosed in U.S. Pat. No. 3,470,084 to Scott is hereby incorporated by reference.
Early FCC units were designed to operate on vacuum gas oils directly fractionated from crude oils. Typically, these vacuum oils came from high quality crude oils. Today, much of the high quality feedstock for FCC units has been depleted and modern FCC units process less favorable materials. These less favorable materials include a substantial amount of sulfur compounds, metal cations, and nitrogen compounds such as ammonia and nitrites. As a result, the contaminant levels in the FCC effluent have been growing, particularly in the C.sub.3 -C.sub.5 effluent fraction. Without appropriate treatment, the contaminants in the C.sub.3 -C.sub.5 FCC effluent fraction can be transmitted to sensitive downstream processes where they reduce the effectiveness of downstream catalysts and create unfavorable by-product reactions in processes such as etherification. The most important function of a water wash system in the FCC gas plant or gas concentration section is to remove solids and thereby prevent the build-up of solids in the fractionation, cooling, and rotating equipment. Solids and their deposit in the equipment lead to corrosion under these deposits. The presence of the ammonia, the hydrogen sulfide, and hydrogen cyanide can lead to the formation of ammonium bisulfide and cyanide. When the concentration of cyanides is high, corrosion and hydrogen blistering may potentially occur in the gas concentration section. Some of these problems can be avoided by keeping the impurities in dilute concentrations or by purging the maximum amount of these impurities as well as any ammonium bisulfide and cyanide from the system in the water withdrawn from the gas concentration unit. However, it is also highly desirable from an environmental aspect to minimize the amount of wash water employed to purge these impurities.
One type of water wash scheme which is most favored in the industry and widely practiced commercially is known as the concurrent water circulation process. In the concurrent water wash scheme, the wash water is circulated in the direction of increasing pressure and a waste water stream is withdrawn at the highest pressure. For example, in the concurrent water wash scheme, the wash water is admixed with the overhead stream from the FCC main column at the lowest pressure in the gas concentration unit. The overhead stream is cooled and condensed to provide a low pressure water stream and a low pressure vapor stream. The low pressure vapor stream is compressed to an intermediate pressure, washed with the low pressure water stream, cooled, and flashed to provide an intermediate vapor stream and an intermediate liquid stream. The intermediate liquid stream is compressed to a high pressure, admixed with the intermediate liquid stream, cooled, and condensed to provide a high pressure liquid stream, a high pressure vapor stream and a high pressure water stream. A portion of a high pressure water stream is withdrawn as a sour water waste stream comprising ammonium bisulfides and impurities, and a portion of the high pressure water stream is recycled to be admixed with the overhead stream. This wash water cascade scheme is referred to as "Option b" in response to "Question 26" of the "1994 National Petroleum Refiners Association (NPRA) Q & A Session on Refining and Petrochemical Technology," pages 78-80, 1994, published by the NPRA, and hereby incorporated by reference. Wash water rates for this scheme range from about 1 to 25 gallons per minutes per thousand barrels of total feed to the FCC reactor.
Other wash water cascade schemes, referred to as "Option a" and "Option c" in the above mentioned NPRA reference, introduce the fresh wash water at intermediate pressure and high pressure and remove the sour water waste stream at low pressure. At high pressure, the wash water is in equilibrium with the hydrocarbons and when the sour water stream is withdrawn at high pressure as in option b, there is a small amount of hydrocarbon gas dissolved in the high pressure sour water stream. To avoid sending this gas to downstream treatment facilities which can not tolerate the presence of the hydrocarbon gas, the high pressure sour water stream is returned to main column overhead stream where the hydrocarbon gas can be flashed off at low pressure and the sour water stream is withdrawn at low pressure. However, the impurities in the water withdrawn at the high pressure are thereby reintroduced to the low pressure overhead stream. These schemes result in returning to the main column overhead stream the ammonia, H.sub.2 S, and HCN which were concentrated at the high pressure. Because hydrocarbon gases dissolved in the high pressure water stream can build up and be carried out with the sour water stream in "Option b" schemes, "Option a" schemes are favored to avoid producing a hydrocarbon gas stream in the downstream sour water treatment facilities. However, the wash water rates for the Option a and c schemes are about the same as those of the "Option b" scheme.
Water cascade schemes for FCC gas concentration units are sought which permit the operation of the gas concentration at lower water rates which produce less waste water while controlling the build-up of impurities in the gas concentration unit.